There are numerous applications for the cleaning of sensitive components, such as spacecraft components, bearings, and electronic equipment. Electronic or electrical components can become contaminated through usage, e.g., by smoke, dust, and other airborne contaminants, or by oils or lubricants. Oils are more difficult to displace than many other contaminants due to their lower surface tensions and higher viscosities, which make them difficult to remove with many solvents and/or detergents.
A number of alcohols, fluorinated alcohols and other halogenated compounds have been found to be effective as displacing agents for contaminants, particularly oily contaminants. For example, chlorinated hydrocarbons and chlorofluorocarbons (CFCs), such as Freons(trademark), are commonly used. Concentrated corrosive acids or bases have also been used as cleaning agents. These reagents are often costly, hazardous to handle and present environmental and disposal problems.
Sonic cleaning has been used for decontaminating and/or disinfecting instruments used in medical, dental, surgical or food processing, for example. This method generally involves placing the instruments in an aqueous bath and treating them with ultrasonic energy. Treatment with ultrasonic energy has long been recognized to be lethal to microorganisms suspended in a liquid, as described, for example, by Boucher in U.S. Pat. No. 4,211,744 (1980). Ultrasonic energy has also been used for cleaning and sterilizing contact lenses (U.S. Pat. No. 4,382,824 Halleck (1983)), surgical instruments (U.S. Pat. No. 4,193,818, Young et al. (1980) and U.S. Pat. No. 4,448,750 (1984)) and even body parts, such as a doctor""s hands (U.S. Pat. No. 3,481,687, Fishman (1969)).
After fluid processing, the components normally need to be dried. Evaporation of rinsing liquids is not desirable since it often leads to spotting or streaking. Even the evaporation of ultra high purity water can lead to problems when drying on the surfaces of some components. For example, such water can dissolve traces of silicon and silicon dioxide on semiconductor surfaces, and subsequent evaporation will leave residues of the solute material on the wafer surface.
A device known as a spin-rinser-drier is useful for drying objects without water evaporation. These devices utilize centrifugal force to xe2x80x9cthrow xe2x80x9d the water off the surfaces of the object. This can cause breakage because of the mechanical stress placed on the object, particularly with larger or fragile objects. In addition, contamination control is problematic due to the mechanical complexity of the spin-rinser-drier. Since the objects conventionally travel through dry nitrogen at a high velocity, static electric charges can develop on the surface of the object. Oppositely charged airborne particles are then quickly drawn to the object""s surface when the drier is opened, resulting in particulate contamination. Finally, it is difficult to avoid evaporation of water from the surface of the object during the spin cycle with the attendant disadvantages discussed above.
More recently, methods and devices have been developed for steam or chemical drying of sensitive objects. Chemical drying generally comprises two steps. First, the rinsing fluid is driven off and replaced by a non-aqueous drying fluid. Second, the non-aqueous drying fluid is evaporated using a pre-dried gas, such as nitrogen. A method for chemically drying semiconductor wafers using isopropanol is described in U.S. Pat. No. 4,778,532, and in U.S. Pat. No. 4,911,761.
It is an object of the present invention to provide a process and apparatus which can be used for degreasing, cleaning and drying of sensitive components, particularly components having complex configurations.
The present invention relates to methods and apparatus for cleaning the surface of an object by placing the object in an enclosed vessel and sequentially passing cleaning and/or rinsing fluids through the vessel, then drying the object under conditions which do not permit the deposition of residues on the surface of the object. The cleaning and rinsing fluids are selected based on the type of contamination to be removed and can include aqueous and non-aqueous fluids. In a preferred embodiment, sonic energy is applied to at least one of the fluids in the vessel.
The process is particularly useful for cleaning sensitive electronic components, such as complex parts, e.g., reading heads used in computer systems for reading and/or recording information on disks. The process is useful for cleaning hard disks, aerospace parts (e.g., gyroscopes, ball bearings), medical devices and other precision parts. The process can be used to deflux printed circuit boards, and for degreasing microparts, in particular, as a replacement for traditional Freon(trademark) processing. Components having numerous interfaces and facets, that is, which are involuted, can be thoroughly cleaned and dried using the present method. The present protocols can be used on metallic, ceramic or plastic surfaces.
The apparatus comprises an enclosure for enclosing the object to be cleaned, and means for passing a flow of liquid though the enclosure and around the object disposed therein. Cleaning and rinsing liquids are preferably introduced into the vessel through a port located in the bottom of the vessel. The apparatus may include a means for agitating the liquid to permit thorough cleaning or rinsing of all surfaces. Preferably a means for generating sonic waves, which can be ultrasonic or megasonic energy, is used for this purpose. The apparatus optionally can contain spray heads for pre-cleaning the object by spraying it with a liquid to remove gross contaminants. The apparatus contains a means for removing the liquid from the enclosure which can be a second port located at the top of the vessel, and means for drying the object by filling the vessel with an organic drying solvent or vapor.
In a preferred embodiment of the invention, means for introducing inert gas or air and means for circulating the washing or rinsing liquids through the vessel are included in the apparatus. The vessel preferably comprises a port at its top so that a fluid in the vessel can be vented out the top port while a second fluid is introduced into the vessel through the bottom port. Vapor or gas is introduced through an inlet at the top to displace a fluid downwardly through the bottom. This allows one fluid to be directly replaced with another fluid without exposing the objects to air. The two ports may be connected via a line, thereby permitting a fluid to be circulated through the vessel. The apparatus preferably includes means for supplying the vessel with a washing or rinsing liquid without exposing the fluid to the air. In one embodiment, a storage tank containing the liquid is connected to the vessel via a line. The storage tank may be supplied with a means for pressurizing the tank, for example, with an inert gas. The washing or rinsing liquid is then returned to the tank after use. In another embodiment, the apparatus contains means for filtering, distilling or otherwise recycling the liquids for reuse in the present system.
The method of the invention generally involves the following steps: placing the object to be cleaned in the vessel and sealing the vessel; filling the vessel with a washing fluid to immerse the object and contact all of the surfaces of the object with the fluid; preferably, agitating the liquid using sonic energy or other agitating means; filling the vessel with a rinsing fluid to displace the washing fluid and to immerse the object; and removing rinsing fluid from the surfaces of the object using an organic drying solvent under conditions such that substantially no rinsing fluid droplets, cleaning agents or contaminants are left on the surfaces of the object after removal of the rinsing fluid. The vessel can be purged with an inert gas, such as nitrogen, and/or with air, prior to removing the object from the vessel.
In one embodiment of the method, the object of interest is cleaned using an aqueous or semi-aqueous protocol. In this embodiment, the object is immobilized in the enclosure and, optionally, prerinsed by spraying the object with water. The enclosure is then filled with rinse water to remove mechanically displaced surface contaminants or gross particulates. In the aqueous protocol, the object is then immersed in a cleaning solution comprising a water/surfactant mixture. In the semi-aqueous protocol, the cleaning liquid is preferably a hydrocarbon solvent/surfactant mixture. Ultrasonic or megasonic energy can be applied through the liquid medium if desired or needed. The resulting agitation allows even involuted or hard-to-reach surfaces of the component to be thoroughly cleaned. The parts remain stationary while the cleaning and rinsing fluids move around them. The component is rinsed again with water to remove the surfactant. In a preferred embodiment, the final rinse is followed by a drying step in which a water-miscible organic vapor, e.g., alcohol or acetone vapor, is injected into the vessel. The organic vapor drives the water from all surfaces of the component. The vessel containing the alcohol-dried component can then, optionally, be purged with nitrogen and/or air prior to removing it from the vessel. This ensures that all surfaces of the object are thoroughly dried and residue-free.
In another embodiment of the method, the object of interest is cleaned using a non-aqueous protocol. The object is immobilized in the enclosure and, optionally, prerinsed with water or an organic solvent to remove gross particulates. The object is then immersed in an organic cleaning solvent, preferably a terpene or mixture of terpenes. The terpene solvent optionally can contain a surfactant. Ultrasonic or megasonic energy is applied if necessary or desirable. The cleaning solvent is then drained from the vessel, and the vessel is filled with a rinsing solvent which solubilizes residual cleaning solvent and removes it from the surfaces of the object. This rinsing step can be followed by drying with hot organic vapor. The vessel is then purged with an inert gas which thoroughly dries the object before it is exposed to air.
The method and apparatus are particularly useful for ultracleaning of objects which must be as free as possible of contamination. The combination of precise control of solvent, washing and rinsing reagents, hydraulically full flow, ultrasonic or megasonic energization and removal of rinse droplets and/or contaminants with a drying solvent or vapor permits extraordinarily thorough cleaning and rinsing to produce essentially contaminant-free surfaces. The results achieved through use of the apparatus and process of the invention is referred to hereafter as xe2x80x9cultracleaningxe2x80x9d.
The present apparatus and method incorporates many desirable features for cleaning sensitive electronic components, ball bearings, printed circuit boards, medical devices, hard disks for computers and precision parts. The apparatus and method can be used to thoroughly clean and/or decontaminate the surfaces of objects containing many small parts, involuted surfaces or having a highly complex configuration. The reaction vessel is a totally enclosed environment, therefore contact of a human operator with aggressive cleaning solvents or solvents having a strong odor, such as terpenes, is eliminated. The use of terpenes is particularly advantageous in that terpenes are naturally occurring, biodegradable, and are excellent solvents for most contaminants. Terpenes can be used for cleaning objects which traditionally required the use of Freons, which are costly and environmentally harmful. The odor associated with most terpenes is not problematic because the system is completely enclosed.
The objects to be treated are immobilized in the vessel, so fragile or sensitive parts can be cleaned with no product movement. Non-aqueous solvents can be recycled for repeated reuse. The apparatus and method provides a combined cleaning and drying tool, thereby reducing equipment cost, minimizing product movement and exposure to chemicals. The method eliminates harmful gas-liquid interfaces, which can result in flash corrosion and/or staining, and protects the cleaned product from sources of external contamination. The method can be adapted for automated chemical handling and comprehensive computer integration of the process.